1. Field of the Invention
The present invention relates to an optical transmission device for use in optical communications.
2. Background Art
Various techniques are known for optical communications using optical transmitters and receivers, as disclosed, e.g., in Japanese Laid-Open Patent Publication Nos. 11-098122 (1999), 11-251674 (1999), 2007-059537, 2006-033581, and 2005-341529 and Japanese Domestic Republication of International Patent Application No. WO 2004/102754. The combination of an optical transmitter and receiver is generally referred to as an optical transceiver (or OTR).
An optical transmitter includes a modulator and a semiconductor laser to modulate an electrical signal onto an optical signal. The wavelength of the output beam (hereinafter also referred to as the “output wavelength” for convenience) of the optical transmitter need be accurately controlled to provide high-quality optical communications. Since optical communications systems using wavelength division multiplexing (WDM) each transmit information on different wavelengths of light simultaneously, it is especially important that these systems adjust the output beam of each optical transmitter to the desired wavelengths, i.e., perform an accurate wavelength control. It should be noted that the wavelength control of the output beam is more important with dense WDM (or DWDM), since this technology uses more closely spaced wavelengths than WDM.
When it is necessary to control the wavelength of the output beam of an optical transmitter, it is common that the transmitter is provided with means for detecting the output wavelength. The value of the detected output wavelength can be used, e.g., by the external system host as information for preventing various system failures. In order to enable this, it is common practice that the optical transmitter supplies information about the output wavelength to the system host or other external devices, as necessary. The means for detecting the output wavelength (hereinafter also referred to as the “wavelength detecting system” for convenience) has been implemented in various practical forms. Generally, a wavelength detecting system includes various types of components for monitoring the output beam of the optical transmitter, such as beam splitters, optical filters, and photodetectors, for example.
In some practical applications it is necessary to correct the output wavelength value detected by the wavelength detecting system. This is because some error is inevitably introduced during the detection process in which the sampled light is processed and measured by the components of the wavelength detecting system. The correction calculation involved complicates the control of the optical transmitter. This problem is especially acute with wavelength detecting systems using an etalon, since etalons have complicated wavelength transmission characteristics. Thus, wavelength detecting systems using an etalon require a complicated correction computation.